a. Field of the Invention
This invention relates to earth boring tools, and more particularly to drill string resilient units useful in earth boring by the rotary system of drilling, such tools sometimes being called vibration dampers or shock absorbers, and relates specifically to a drill string splined resilient tubular telescopic joint.
b. Objects of the Invention
A principal object of the invention is to provide such a tool especially adapted for balanced load drilling, i.e. where the drilling weight and pump apart force are equal. Though the tool is specially intended for balanced load drilling, the tool may also be used with advantage under such other load conditions as may normally be expected and includes features of general utility in drill string resilient units.
Other objects and advantages of the invention will appear as the description thereof proceeds.
c. Description of Certain Prior Art
It appears that many prior art drill string resilient units have not been intended for use in balanced load drilling. For example, in U.S. Pat. No. 2,991,635--Warren, there is disclosed a tubular telescopic tool employing splines to transmit torque and multiple helical springs disposed in parallel to transmit axial force. Three sets of multiple sliding seals are employed, one above and one below the spline-spring means, and one separating the spline from the springs. The uppermost multiple seal includes upper and lower pairs of O-rings and a lubricating packing therebetween, with a grease fitting to allow injection of grease. However, the spring means come into action only upon contraction of the damper from the no load condition. Warren states that:
"normally the springs 60 will be designed to be compressed substantially half way during a normal drilling operation. However, if a greater or lesser weight is desired on the drill bit 30, the drill string may be lowered or raised a short distance to increase or decrease the compression of the springs 60," (col. 5, lines 50-60). PA1 "As may occur in some cases, the area enclosed by the outside diameter of the pressure seal ring times the differential pressure may be in excess of the load to be carried on the bit. In this case the damper would remain pumped open and would not function as intended." PA1 "Drilco Industrial SHOCK SUB.RTM. Vibration Dampener" bearing the notation "0976 Printed in U.S.A." PA1 U.S. Pat. No. 4,139,994 issued Feb 20, 1979 to George Abraitys Alther. PA1 "Vertical shock is absorbed by a large elasomeric element in compression. The element material is specially compounded to provide optimum characteristics of load carrying ability, fatigue resistance, resilience and dampening. The element is geometrically shaped to provide uniform "softness" at bit weights from near zero to 30,000 lbs. A second, smaller elastomeric element is provided to cushion reverse loading during severe rebounds or when pulling from the hole."
In U.S. Pat. No. 3,949,150--Mason et al, there is disclosed a sealed, lubricated, splined, resilient, tubular telescopic joint for a drill string wherein contrary to the more usual arrangement the mandrel is connected to the drill string and the barrel or case is connected to the bit. A stack of rubber rings each sandwiched between flanged metal rings serves as a resilient element. Referring to the prior art it is said that due to hydrostatic pressure and the difference in area between the upper and lower seals of some dampers the resilient element is preloaded in compression, necessitating the use of "hard" deformable elements e.g. with an initial spring rate requiring 100,000 lb. for the first 3/4 inch deflection and a later spring rate requiring another 100,000 lb. for the next 1/4 inch deflection, but which will still go solid at normal drilling depths due to hydrostatic pressure. Other dampers are said to avoid this by using a floating seal for one end of the spring chamber, so as to equalize the pressure, but in such dampers Mason indicates that inappropriate springs were used.
Mason notes that for many shock absorbing elements the spring rate increases with the load causing the damper to operate at a point where the spring rate is high. Mason's objective is to provide a damper which operates at a position where the spring rate is low at all times. Mason therefore discloses a shock absorber having a low initial spring rate and having a floating seal to equalize pressure and eliminate preload. But since his spring means is only single acting, this requires operation about a mean position in which the spring means is partially compressed, so that the spring rate is higher.
Mason contemplates operating the damper with a static spring force of about 25,000 lb., at which point the damper has a spring rate of 20,000 lb. per inch. This static load on the spring is said to be the difference between 55,000 lb. drilling weight and a 30,000 lb. pump apart force. For operating in shallow wells where the bit weight may be low, to insure some static compression of the spring Mason proposes to reduce the pump apart force by reducing the area of the seal between mandrel and barrel. To that end he provides a lower seal ring of smaller area and vents the annulus between the lower seal ring and the floating seal ring.
Mason further notes (at col. 12, lines 21 et seq) that:
To overcome this situation Mason eliminates the static pressure balancing floating seal and under-fills the spring chamber with lubricant so that the static pressure overcomes the pump apart force and compresses the spring enough for it to be operative.
In short, Mason et al disclose a single acting damper intended to operate about a partially compressed static load position thereby to avoid banging against the travel limit stops upon imposition of dynamic loads.
The Drilco Industrial division of Smith International, Inc. manufactures a vibration damper including telescoping members with a shoulder on the mandrel in between two shoulders on the barrel forming upper and lower pockets in which are disposed shaped rubber sleeves to provide variable spring rates. The upper sleeve is effective during normal operating conditions and the lower sleeve is effective when the bit is lightly loaded and the pump apart force exceeds the bit weight causing the damper to be extended rather than contracted in the static condition. The damper is intended especially for water well and other shallow hole drilling wherein the pump apart force is small. The lower sleeve is different from the upper sleeve, whose spring rate increases much less rapidly with deflection. Only one or the other of the rubber sleeves is strained at any one time. The spline-spring chamber is exposed to drilling fluid. This construction is shown in a brochure entitled:
and in
It is stated in the brochure: